1. Field of the Invention
This invention relates to operation of gas turbine engine laser shock peened airfoils and, more particularly, to extending tolerances of FOD damaged laser shock peened airfoils.
2. Discussion of the Background Art
Leading and trailing edges of aircraft gas turbine engine rotor blade airfoils are subject to wear and damage that forms gouges and cracks due to ingestion of material into the engine, this is often referred to as foreign object damage (FOD). The airfoil is also subject to vibrations generated during engine operation and the cracks due to FOD operate as high cycle fatigue stress risers producing additional stress concentrations around them. These cracks tend to grow and the airfoils must be periodically inspected. Conventionally, airfoils having cracks about 20-50 mils long (0.02-0.05 inches) or greater are repaired using a process known as blending. This allows the service life of the blade or vane to be extended saving the time and cost of replacing the part. Procedures and tools have been developed to accomplish this, see for example U.S. Pat. Nos. 5,197,191, 5,281,062, and 5,644,394. FOD inspection and blending repair procedures are typically distributed to aircraft gas turbine engine customers and repair shops. Inspection procedures, typically require visual inspection for cracks in the airfoil leading and trailing edges and have established crack length limits, typically measured from the edge inward and typically in the range of about 20-50 mils long. Blending is an expensive and time consuming process and, therefore, it is highly desirable to extend the service of blades and vanes having FOD damage that would otherwise be repaired with blending or scrapped. There are many reasons to extend the useful life of the airfoils and avoid such a repair procedure. With the advent of blade integral disks or BLISKs, there is an even greater need to avoid the blend repair of the leading and trailing edges of the blade airfoils because blisk blades cannot easily be disassembled or repaired and BLISK replacement is very expensive.
The present invention includes a method for operating a gas turbine engine airfoil having leading and trailing edges subject to FOD damage. The method includes inspecting the airfoil along at least a first one of the edges having at least a first laser shock peened patch along the first one of the edges, ascertaining FOD lengths of foreign object damage (FOD) extending from the first one of the edges into the airfoil within the patch, and allowing the airfoil to remain in service if the FOD lengths are up to a maximum length of about one-half a width of the laser shock peened patch as measured from the edge. In one embodiment, the width of the laser shock peened patch is in a range of about 0.50-1.50 inches. In another embodiment, the airfoil is allowed to remain in service if the maximum length is in a range of about 0.150-0.300 inches.
Yet another method of the present invention includes operating a gas turbine engine airfoil having leading and trailing edges subject to FOD damage and for which there is a first limit on length of foreign object damage (FOD) as measured from a first one of the edges of a corresponding non-laser shock peened airfoil. The method includes the following steps: A) inspecting at least a first one of the edges having at least one laser shock peened patch along the first one of the edges, B) ascertaining FOD lengths of foreign object damage (FOD) extending from the first one of the edges into the airfoil within the patch, and C) allowing the airfoil to remain in service if the ascertained FOD lengths from step B) are up to about five to eight times the first limit on length of foreign object damage as measured from the leading edge.
Another method includes laser shock peening the leading edge and forming pressure and suction side laser shock peened patches on pressure and suction sides, respectively, of the airfoil along the leading edge such that the laser shock peened patches extend along substantially an entire spanwise length of the leading edge. Then operating the gas turbine engine airfoil in service, inspecting the airfoil, ascertaining FOD lengths of foreign object damage (FOD) extending from the leading edge into the patches, and allowing the airfoil to remain in service if the FOD lengths are up to about one-half a width of the laser shock peened patches as measured from the leading edge. One more particular method of the invention includes laser shock peening using dual laser beams fired simultaneously with each of the laser beams having a power in a range of about 40-60 joules and forming laser beam spots with a diameter of about xc2xc inch at a surface of the patch. In one more particular embodiment, each of the laser beams having a power of about 50 joules and in another 40 joules.